1. Field of the Invention
The present invention relates to a surface emitting laser, a method for manufacturing a surface emitting laser, and an image forming apparatus.
2. Description of the Related Art
A vertical cavity surface emitting laser (VCSEL) which is one of surface emitting lasers is capable of extracting light in a direction vertical to a surface of a substrate. Therefore, a two-dimensional array can be easily formed.
High density and high speed can be realized by parallel processing with a plurality of beams emitted from the two-dimensional array, and various industrial applications such as optical communication and the like are expected. For example, when a surface emitting laser array is used as an exposure light source of an electrophotographic printer, an image forming process with a plurality of beams can be improved in density and speed.
Electrophotographic applications of the type described above require the formation of stable and micro laser spots on a photosensitive drum. Accordingly, stable operations in a single transverse mode and a single longitudinal mode are required laser characteristics of a VCSEL.
In surface emitting lasers, in order to improve performance, there has been developed a method for injecting a current only in a necessary region by forming a current confinement structure using a selective oxidation technique.
This method for forming a current confinement structure includes providing an AlGaAs layer (for example, Al0.98Ga0.02As) having a high Al composition ratio in a multilayer mirror and selectively oxidizing the layer in a high-temperature steam atmosphere to form the current confinement structure. Since an oxidized region is converted from a conductive region to an insulating region, a current can be injected into a desired portion of an active layer region.
In order to achieve high output in a selective oxidation-type VCSEL, it is necessary to increase the diameter of an aperture serving as a conductive region of a current confinement structure. However, in a distribution of current carriers, the carriers are concentrated in the edge portion of the aperture, which is a boundary between a conductive region and an insulating region. Therefore, when the diameter of the aperture is increased, high-order transverse mode oscillation with light intensity highly distributed to the edge portion is easily generated.
In an attempt to solve the problem of carrier concentration at the edge portion of the aperture, a method of using two current confinement structures is disclosed by H. J. Unold et al., in “Large-Area Single-Mode Selectively Oxidized VCSELs: Approaches and Experimental”, Proceedings of SPIE Photon, West, Vol. 3946 (2000), pp. 207-218, (hereafter “the Unold document”). To illustrate the method of using two current confinement structures, FIG. 10(b) of the Unold document is shown in FIG. 9 herein.
In the method using two current confinement structures, as illustrated in FIG. 9, a current confinement structure 930 having a smaller aperture diameter than a current confinement structure 920 disposed near an active layer 910 is disposed further away from the active layer 910 than the current confinement structure 920. As a result, charge carriers are concentrated in a central portion of the aperture in the current confinement structure 920 provided nearer to the active layer 910. Since the current confinement structure 920 provided nearer to the active layer 910 controls the mode of resonant light, the efficiency of coupling of the carriers and fundamental mode light is enhanced by injecting the carriers in the central portion of the aperture. Therefore, the use of two current confinement structures can suppress high-order mode oscillation and form high-output surface emitting lasers as compared with use of one current confinement structure.
In order to achieve a single transverse mode, effective coupling of carriers and the fundamental mode light is required. Therefore, in the technique of providing two current confinement structures as described in the Unold document, it is necessary that the aperture diameter of the current confinement structure away from the active layer is smaller than that of the current confinement structure near the active layer.
For example, when the aperture diameter of the current confinement structure 920 disposed near the active layer 910 is 6 to 7 μm, the aperture diameter of the current confinement structure 930 disposed away from the active layer 910 is about a half, i.e., about 3 to 4 μm.
In addition, U.S. Pat. No. 5,493,577 (hereafter “the '577 patent) describes that two oxidized regions constituting two current confinement layers may be the same or different (see, e.g., column 15). The '577 patent also describes that different oxidized regions can be formed by controlling an Al composition; and that different oxidized regions can be formed by using a stepped mesa.
As described above, the Unold document discloses that two current confinement structures are provided, and the aperture diameter of the current confinement structure away from the active layer is smaller than that of the current confinement structure near the active layer.
The '577 patent describes that different oxidized regions can be formed by controlling an Al composition ratio of an oxidizable layer which becomes the current confinement structure. Since the oxidation rate increases as the Al composition ratio of a semiconductor increases, when semiconductor layers having different Al composition ratios are oxidized for the same time, a semiconductor layer having a higher Al composition ratio has a smaller aperture diameter than a semiconductor layer having a lower Al composition ratio.
However, the inventors of the present invention confirmed that from the viewpoint of reliability of a device, a problem is present in a surface emitting laser having a plurality of current confinement structures formed by controlling the Al composition ratio of an upper oxidizable layer to be higher than that of a lower oxidizable layer.